The invention relates to a yarn brake and to a yarn feeding device.
In a yarn brake known from DE-U-94 06 102 which is to be mounted at a yarn feeding device, the braking body is a thin walled metallic ring band with the shape of a frustocone jacket the inner side of which forms a braking surface co-operating with the withdrawal rim of the storage drum. The braking body is supported at the outer edge region of the carrying structure by means of a frustoconical plastic foam ring having a rectangular cross-section. Said plastic foam ring is glued to the rear side of the braking body and into the annular carrying structure. The carrying structure is supported at the stationary support of the yarn feeding device by a group of axial coil springs located coaxially to the centre axis of the yarn brake. The foam material forms a first spring assembly and allows the braking surface to locally yield in the yarn passing region. The foam material also has to transfer axial load from the axial coil springs onto the braking surface. The carrying structure is rigid. The first spring assembly is acting substantially at the diameter of the braking surface. The necessary bondings or gluings are difficult to manufacture and tend to degrade during operation of the yarn brake. Furthermore, any gluing spots cause inhomogeneties which produce fluctuations of the yarn tension during the orbiting movement of the withdrawn yarn between the braking surface and the withdrawal rim. A correct centring of the braking body at the withdrawal rim also can not be achieved for operation conditions.
It is an object of the invention to provide a yarn brake of the kind as mentioned and a yarn feeding device with a yarn brake which is easy to manufacture and very reliable, at which the braking surface reliably remains centred and wherein the braking surface shows an extremely uniform deformation performance in the circumferential direction. At the yarn feeding device the yarn brake should lead to a braking effect which fluctuates as little as possible. Furthermore, the braking effect should be accompanied by a self compensation effect. This means that the tension in the withdrawn yarn increases as little as possible in case of increasing yarn speed or increasing yarn acceleration, respectively, such that the yarn tension only varies a little relative to a precisely adjustable basis tension. Finally, a large adjustment range should be achieved within which the braking effect can be adjusted gradually.
The above object is achieved by providing a yarn brake having an annular and thin-walled braking body with a circumferentially continuous conical braking surface. The braking body is mounted via a first spring assembly in an annular carrier structure arranged at a side of the braking body facing away from the braking surface, which carrier structure is mountable at a stationary support. The conical braking surface of the braking body is continued unitarily by a plurality of springy lamellas extending outwardly essentially in a star-like fashion. The lamellas are formed with a pre-shaped curvature and are interconnected at a radial distance from the braking surface by an outer ring element and the lamellas constitute the first spring assembly within said braking body.
The above object is also achieved by providing a yarn feeder having a yarn brake, the yarn feeder including a storage drum forming a frontal withdrawal rim which is continuous in the circumferential direction. The yarn feeder also includes a housing bracket with a stationary support for the yarn brake, and the yarn brake includes an annular, thin-walled radially deformable braking body with a circumferentially continuous conical braking surface. The braking body is mounted via a first spring assembly in an outer ring of a cup-shaped, hollow carrier structure supported at the stationary support so as to axially and yieldably press the braking surface against the withdrawal rim. The carrier structure is tiltably supported by a universal joint at the stationary support, the universal joint centre essentially coinciding with the centre axis of the storage drum, and the universal joint consists of two positively interengaging ring parts, one of which is defined by a seat ring of the carrier structure and the other is a ring-shaped counter-engagement element formed with a spherical bearing surface.
The yarn brake can be manufactured for fair costs and in a simple way, since the braking body having the integrated and pre-bent lamellas and the ring element interconnecting the outer ends of the lamellas and the annular braking surface can be prefabricated comfortably. No bondings or gluing spots are needed to manufacture the braking body or to fix the braking body in the carrying structure. The lamellas generate their spring effect between the inner and outer ring parts and in the circumferential direction uniformly and with a large effective lever arm. The axial pre-load is transferred into the braking body by the carrying structure which in turn is supported at the stationary support. A long-stroke elasticity range results as well as a very uniform deformation performance of the braking surface in the circumferential direction. A correct centring of the braking surface at the withdrawal rim also results. Thanks to the long-stroke of the spring assembly, the braking surface compensates for possible deviations of the roundness of the withdrawal rim. Since there is no foam materials or elastomeric materials and no bonding spots, spring properties and braking properties can be assured which remain constant for long operation durations and which are not influenced by ageing or degrading. Furthermore, it is assured that deviations from a precise circular shape of the withdrawal rim do not influence the yarn tension or the uniformity of the braking effect over a full revolution of the yarn withdrawal point, as such deviations are compensated for. A particular advantage of the structure of the braking body is a relatively flat spring rate which allows precise and gradual adjustment of the braking effect between an extremely weak braking effect and an extremely strong braking effect. This is due to the relatively long axial stroke between the braking surface positioned by the pre-bent lamellas in a predetermined orientation and the carrying structure. The positive effect results from the fact that the pre-bent lamellas distribute over a long adjustment stroke the adjustment range of the yarn brake.
The carrying structure is supported by the universal joint in the yarn feeding device such that it can tilt to all sides. Due to the form fit of the ring parts the universal joint leads to a reliable centring. The universal joint does not allow any uncontrolled relative lateral movements but an optimum tilted position of the carrying structure to compensate for misalignments between the support and the storage drum of the yarn feeding device. Such misalignments may result from the assembly or the manufacturing of the components. The braking body itself is an easily replaceable part.
Manufacturing is facilitated also when the outer ring element is a unitary part of the braking body. The braking body for example is made from a metal foil blank, e.g. by etching or laser cutting the interspaces between the lamellas before they undergo a plastic deformation into their curved configuration. By this measure a first spring assembly with a long stroke is integrated into the braking body. The first spring assembly additionally holds the braking surface in an orientation at the withdrawal rim needed for an optimum braking performance. Due to this measure, the braking surface need not be deformed by the axial pre-load out of a radial orientation which is particularly advantageous in case of a very weak braking effect, e.g. for very thin yarns.
Alternatively, the outer ends of the lamellas are imbedded in a plastic ring, preferably by injection moulding, such that the plastic ring becomes part of the braking body.
The braking body is seated with a form fit in a socket of the carrying structure and is detachably mounted in the socket by means of a securing ring. The securing ring, preferably made from plastic material, is only needed for transport or the assembly procedure to prevent detachment of the braking body. In the operational position of the yarn brake, the braking body is secured by the axial pre-load.
In order to achieve a very uniform deformation performance of the braking surface and a relatively soft braking body, the stretched length of each lamella ought to correspond to a multiple of the width of the braking surface. Small interspaces between adjacent lamellas prevent the yarn from being caught.
The lamellas are pre-bent almost or exactly to an essentially radial orientation, or even further. Their inner roofs continue along the generatrice of the conical braking surface. Expediently, the ring element of the braking body is positioned radially which simplifies the fixation of the braking body in the carrying structure. By the plastic pre-bending of the lamellas prior to assembly of the yarn brake, the long stroke of the soft integrated spring assembly can be predetermined upon demand.
A long adjustment stroke in connection with long effective lengths of the lamellas and a soft characteristic are achieved by several bends (snake line shape) in the lamellas.
In order to vary the softness of the first spring assembly or the effective length of the lamellas, in case of a given radial mounting space for the yarn brake the lamellas can be formed such that they deviate from a radial orientation.
An advantageous axial pre-load transfer principle is selected in view of the desired self-centring of the yarn brake and a desirable xe2x80x9clazyxe2x80x9d operation performance of the braking surface. In the kinematical chain of the force transmission from the support into the braking surface, the force is acting within an axial long stroke of the elasticity range of the yarn brake from relatively far inward first outwardly into the ring element of the braking body, and then from the outer side by means of the lamellas again inwardly into the braking surface. That long force transmitting path is of advantage for the elasticity and is achieved by the structure of the yarn brake even if the useable mounting space for the yarn brake is limited in radial and axial directions.
Of particular advantage are spokes connecting the outer ring with the seat ring of the carrying structure. They form bending spring arms defining an additional large stroke and soft spring assembly in the yarn brake. These two spring assemblies acting in series within the yarn brake lead, e.g. to an adjustment range of extreme magnitude, a good self-centring function, an optimum compensation of misalignments, and an extreme de-coupling of the braking surface which behaves very passively or xe2x80x9cdeadxe2x80x9d in operation, i.e. the braking surface does not act undesirably dynamically. This is particularly important for high yarn speeds. Expediently, the spring rates of the first and second spring assemblies are substantially similar such that both spring assemblies do work during operation. The large stroke of the adjustment range is particularly expedient to gradually adjust the braking effect. In the lateral and torsional directions the carrying structure, however, is relatively stiff. The carrying structure is relatively soft only in the axial and tilting directions.
Advantageously, leaf springs are provided as spokes of the carrying structure, e.g. eight leaf springs are provided in a flat orientation. The number of lamellas in the braking body can amount to 200 or more in order to achieve a uniform force transmission.
Special dimensional relations for the leaf springs are advantageous. The yarn brake then operates softly over a long stroke. The braking surface shows a very uniform deformation performance. Said advantages result from both spring assemblies acting in series and from the long force transmission path from far inwards to the outer region and back into the inner region.
In order to achieve a uniform braking effect in the circumferential direction it is of advantage when the seat ring of the carrying structure is a component of the universal joint which defines the universal joint in co-action with a stationary counter engagement element. By said universal joint the yarn brake is properly centred in a form fit. There is a large degree of freedom for tilting motions to all sides in order to compensate for occasionally existing manufacturing or mounting tolerances between the stationary support and the storage drum.
The main components of the yarn brake are prefabricated components which then are releasably assembled without bonding spots.
In the yarn feeding device, the yarn brake produces only very small fluctuations of the yarn tension in relation to a pre-adjusted basis yarn tension of the withdrawn yarns. The yarn brake operates with an efficient self-compensating effect, meaning that high yarn speeds or strong yarn accelerations do not cause a significant increase of the yarn tension. To the contrary, in operation an optimum yarn tension profile can be achieved which shows almost no fluctuations. The variability of the braking effect departing from a very weak braking effect is distributed over a long adjustment stroke. The basis tension braking effect can be adjusted precisely.
The full extent of the available mounting space between the storage drum and bracket of the yarn feeding device can be used for the force transmission and the wide range elasticity of the yarn brake. Despite the yarn brake mounted therein, the access to the storage drum and to the area between the storage drum and the withdrawal eyelet is hardly restricted.